Gas cleaning system

ABSTRACT

A gas cleaning system for cleaning gases from an incinerator or the like wherein the gas is discharged at a variable rate including a gas cleaning device such as a wet scrubber that requires gas to be fed into the scrubber at a minimum velocity for efficient operation. A delivery duct feeds the gases from the incinerator to the cleaning device and a discharge duct conducts the gas from the cleaning device. A blower forces or induces the gas through the discharge duct. In order to maintain an efficient rate of flow of gas into the cleaning device, a recirculating duct has one end connected with the discharge duct on the downstream side of the blower and its other end connected with the delivery duct, and flow is proportioned between the recirculating duct and the delivery duct in accordance with variations in the velocity of the gas in the delivery duct so that the gas entering the cleaning device is maintained at an efficient rate of flow. The system further includes a temperature responsive modulating valve for controlling the flow of scrubbing liquid into the cleaning device in accordance with variations in the temperature of the gas exiting from the cleaning device so that the supply of water to the cleaning device can be reduced when the temperature drops and can be increased as necessary when the temperature of the gas increases, and thereby maintain constant temperature and gas weight to the blower to prevent overload of the driving device.

United States Patent DeLisio et al.

[ 1 Oct. 3, 1972 [54] GAS CLEANING SYSTEM [72] Inventors: Michael P. DeLisio, Birmingham; Robert L. Clement, Farmington,

both of Mich.

[73] Assignee: Arco Industries Corporation,

Detroit, Mich.

[22] Filed: Oct. 27, 1970 [21] App]. No.: 84,403

[52] US. Cl. ..55/225, 55/226, 55/227, 55/338 [51] Int. Cl. ..B0ld 47/06 [58] Field of Search ..55/l8, 20, 21, 89, 94, 210, 55/223, 225-227, 338

[56] References Cited UNITED STATES PATENTS 3,239,999 3/1966 Price ..55/338 X 2,197,595 4/1940 Schneible ..55/338 X 2,785,879 3/1957 Cramp ..55/21O X 3,395,510 8/1968 Barnes ..55/2O 3,200,606 8/ 1965 Hewett et al ..55/18 X 3,559,379 2/1971 Lambert ..55/223.

FOREIGN PATENTS OR APPLICATIONS 924,739 5/1963 Great Britain ..55/226 Primary Examiner-John Adee I AttorneyMcGlynn, Reising, Milton & Ethington [5 7] ABSTRACT A gas cleaning system for cleaning gases from an incinerator or the like wherein the gas is discharged at a variable rate including a gas cleaning device such as a wet scrubber that requires gas to be fed into the scrubber at a minimum velocity for efficient operation. A delivery duct feeds the gases from the incinerator to the cleaning device and a discharge duct conducts the gas from the cleaning device. A blower forces or induces the gas through the discharge duct. ln order to maintain an efficient rate of flow of gas into the cleaning device, a recirculating duct has one end connected with the discharge duct on the downstream side of the blower and its other end connected with the delivery duct, and flow is proportioned between the recirculating duct and the delivery duct in accordance with variations in the velocity of the gas in the delivery duct so that the gas entering the cleaning device is maintained at an efficient rate of flow. The system further includes a temperature responsive modulating valve for controlling the flow of scrubbing liquid into the cleaning device in accordance with variations in the temperature of the gas exiting from the cleaning device so that the supply of water to the cleaning device can be reduced when the temperature drops and can be increased as necessary when the temperature of the gas increases, and thereby maintain constant temperature and gas weight to the blower to prevent overload of the driving device.

17 Claims, 1 Drawing Figure 18 TO ATMOSPHERE DAMPER 4 1 MOTOR 2 j DAMPER CONTROLLER 5o MoToR INCINERATOR TEMPERATURE RESPONSIVE VALVE 22 CONTROLLER LIQUID FOR SCRUBBING GAS CLEANING DEVICE GAS CLEANING SYSTEM This invention relates generally to gas cleaning systems and is particularly concerned with gas cleaning systems utilizing gas cleaning devices such as venturi scrubbers, cyclonic scrubbers or impingement scrubbers wherein it is necessary to feed gas into the cleaning device at a minimum velocity for efficient cleaning operation.

In many systems and apparatus for the treatment of gas discharged from incinerators, industrial processes, and the like, the gas is fed through a cleaning device operable to separate and remove solids and other impurities from the gas before it is discharged into the atmosphere. Many gas cleaning devices used in such systems require that the gas be fed into the device at a minimum velocity for efficient operation of the cleaning device. For example, with cleaning devices such as venturi scrubbers, cyclonic scrubbers, or impingement scrubbers, the gas is mixed with water or other liquid so that solid particles entrained in the gas are wetted and made heavier by the liquid and are caused to separate from the stream of gas due to the motion of the gas as it passes through the scrubber. If the gas enters the scrubber with insufficient velocity, inadequate or no separation will take place and the solid particles will be discharged from the scrubber along with the gas.

In sewage and waste disposal systems using large incinerators, the gas to be cleaned is discharged at a variable rate from the incinerator. Other industrial processes also produce gas to be treated at a variable rate. It is thus difficult to maintain efficient cleaning of the gas at all times in systems having cleaning devices such as wet scrubbers, as discussed above, since at times, the gas volume to be treated is discharged at less than the minimum velocity or rate of flow required for efficient operation of the cleaning'device. Uncleaned gas is thus frequently discharged from the system resulting in pollution of the surrounding atmosphere.

Another problem encountered in gas cleaning processes and systems is that of discharging gas into the atmosphere which is saturated with liquid when the atmospheric conditions are such that the saturated gas forms a fog that may impair visibility and creates safety hazards near roads and highways. Furthermore, the temperature of the gas to be treated frequently varies over a wide range and is required to be lowered to an acceptable minimum before the treated gas is discharged into the atmosphere. Consequently, the liquid supplied to the cleaning device may serve not only as a scrubbing or cleaning agent to assist in the removal of impurities from the gas, but also as a coolant to lower the temperature of the gas to an acceptable level. The amount of liquid required to be supplied to the cleaning device for cooling the gas when the gas is in its high temperature range may be more than is required for the cleaning operation. Thus, if a liquid supply is maintained at all times sufficient to cool the gas from its highest expected temperature, more of the liquid will be used than is necessary during a large part of the operation since the gas frequently will be at a lower temperature.

An object of this invention is to provide a gas cleaning system and process wherein the flow of gas into a gas cleaning device is maintained at a minimum flow rate for efficient operation even though the gas to be.

cleaned is produced or discharged from an incinerator or the like at a variable rate.

A further object is to provide a gas cleaning system and process wherein the supply of liquid to a gas cleaning device in the system is increased and decreased automatically in accordance with changes in the temperature of the gas discharged from the system.

In accordance with the present invention, the flow rate of the gas entering the gas cleaning device is maintained at a level required for efficient operation of the cleaning device by automatically recycling a portion of the cleaned gas discharged from the cleaning device 1 when the velocity of the gas to be cleaned falls below the required minimum. The cleaned gas is forced back into the delivery duct conducting the gas to be cleaned to the cleaning device to augment the volume rate of flow in the delivery duct to assure adequate velocity of the gas entering the cleaning device.

The objects of the invention are thus achieved by the provision of a system including a gas cleaning device operable to remove impurities from gas fed therethrough with a delivery duct for conducting gas to the cleaning device and a discharge duct for conducting gas from the cleaning device, the gas being forced through the discharge duct by a blower. In order to maintain the flow of gas into the cleaning device at or above the minimum velocity or flow rate required for efficient cleaning operation, a recirculating ducthas one end connected with the discharge 'duct on the downstream side of the blower and its other end connected with the delivery duct. Flow between the recirculating duct and the discharge duct is proportioned by flow control means in accordance with variations in the velocity of gas in the delivery duct in such a manner that when the velocity of the gas in the delivery duct decreases, the flow of gas into the recirculating duct to the delivery duct is increased to bring the velocity up to the desired level, and when the velocity of the gas from the incinerator in the delivery duct increases, the flow in the recirculating duct is decreased and a greater amount of the flow is directed through the discharge duct into the atmosphere. The flow control means includes a damper which moves in one direction to increase the flow through the recirculating duct and decrease the flow through the discharge duct and in the opposite direction to decrease the flow through the recirculating duct and increase the flow through the discharge duct. The damper is actuated by a motor which in turn is controlled by a flow sensitive control operable to cause the motor to actuate the damper in one direction when the velocity of gas in the delivery duct decreases and to cause the motor to actuate the damper in the opposite direction when the velocity of the gas in the delivery duct decreases.

The temperature of the gas discharged from the cleaning device, as well as the amount of water or other cleaning liquid supplied to the cleaning device, is controlled by a modulating valve responsive to changes in the temperature of the gas discharged from the cleaning device. When the temperature of the discharged gas rises, the control valve increases the flow of liquid into the cleaning device, and conversely, when the temperature of the discharged gas decreases, the flow of liquid into the cleaning device is correspondingly decreased. The temperature and weight of the gas to the blower is thus controlled to prevent overload of the blower motor.

Other objects, advantages and features of the invention will become apparent from the following description taken in connection with the accompanying drawing wherein the single figure schematically illustrates a gas cleaning system embodying the invention.

In the drawing, reference numeral indicates a gas cleaning device 10 which may be a wet scrubber of the cyclonic, venturi or impingement type wherein the efi'iciency of the cleaning operation is dependent upon the velocity or rate of flow of the gas fed into the cleaning device. A delivery duct 12 is connected with the cleaning device 10 for conducting gas to the cleaning device 10 from an incinerator 11 or other gas producing and discharging apparatus or system. A discharge duct 14 is connected with the cleaning device 10 for conducting gas from the cleaning device and discharging the cleaned gas into the atmosphere. Gas is caused to move through the discharge duct by a blower 16, and a recirculating duct 18 has one end 19 connected with the discharge duct on the downstream side of the blower and its other end 21 connected with the delivery duct 12 near the cleaning device 10.

The flow of gas is proportioned between the recirculating duct 18 and the discharge duct 14 on the downstream side of the blower by flow control means in accordance with variations in the velocity or rate of flow of gas in the delivery duct 12. The flow control means includes a damper 20, a damper motor 22 and a damper motor controller 24. The damper 20 is movable in one direction to increase flow through the recirculating duct and decrease the flow through the discharge duct, and is movable in the opposite direction to decrease the flow through the recirculating duct and to increase the flow through the discharge duct. For example, movement of the damper 20 toward the right as viewed in the drawing increases the flow through the recirculating duct and decreases the flow through the discharge duct 14. Conversely, movement of damper 20 toward the left in the drawing decreases the flow through the recirculating duct 18 and increases the flow through the discharge duct 14. The motor 22 may be an air operated or electric proportioning motor operable to actuate the damper 20 to move either toward the left or the right in response to signals from the damper motor controller 24. The damper motor controller 24 may comprise a pressure or flow responsive control connected with the delivery duct 12 by a static pressure tap 25. Controller 24 is operable to energize the motor to move the damper toward the right, or in a direction to increase the flow through the recirculating duct 18, when the velocity of the gas in the delivery duct 12 decreases and to actuate the motor 22 to move the damper 20 in the opposite direction, or toward the left in the drawing, when the velocity of the gas in the delivery duct 12 increases. The velocity changes in duct 12 are sensed by the damper motor controller 24 in accordance with changes in static pressure in the delivery duct 12. Thus, if the velocity of the gas flowing from the incinerator into the delivery duct 12 decreases, the static pressure sensed by the controller 24 at tap 25 will increase and the controller 24 will transmit a signal to the damper motor 22 to cause the damper motor to actuate the damper 20 in a direction to increase the flow through the recirculating duct 18 to make up for the decrease in velocity from the incinerator so that the velocity of the gas entering the cleaning device-l0 is maintained at a level sufficient to provide an efficient cleaning operation. Controller 24 may alternatively be actuated by a pressure responsive switch, the pressure differential across an orifice plate, or various other devices for sensing a change in flow or velocity of the gas in duct 12.

As pointed out above, the gas cleaning device 10 may be a wet scrubber, and reference numeral 26 indicates a conduit for conducting scrubbing liquid into the cleaning device for contact with the gas in the cleaning device. The illustrated cleaning device 10 is an impingement scrubber including a perforated impingement tray or plate 29. The gas enters the lower section of the cleaning device 10. The scrubbing liquid enters on top of the impingement tray 29. As the gas passes through the liquid on the top of tray 29, the liquid is atomized at the edges of the perforations 29b, and together with the contaminates, is impinged on the baffles 29a located above each of the perforations. The contaminates drop to the bottom of the cleaning device and are discharged in a slurry through the slurry outlet 10a.

Conduit 26 is controlled by regulating means operable to vary the flow of liquid into the cleaning device in accordance with variations in a condition exterior of the cleaning device. The regulating means comprises a modulating valve 28 and a temperature responsive valve controller 30 for the valve 28 operable to actuate the valve 28 to increase the supply of liquid to the cleaning device 10 when the temperature of gas in the discharge duct 14 increases, and operable to actuate valve 28 to decrease the supply of liquid to the cleaning device 10 when the temperature of the gas in the discharge duct 14 decreases. Consequently, the temperature of the gas discharged from the scrubber or cleaning device 10 can be controlled by increasing the supply of liquid as necessary tocool the gas and conversely, by decreasing the supply of liquid when the temperature is below the maximum temperature desired. It may also be desirable to control the amount of moisture in the gas discharged into the atmosphere from the duct 14 in accordance with atmospheric conditions to prevent the formation of fog which may be objectionable to the surrounding areas.

In the illustrated system, the gas discharged from the incinerator into the delivery duct 12 may pass through a quenching zone 31 wherein water or other cooling liquid is sprayed into the delivery duct 12 to lower the temperature of the gas prior to its entry into the cleaning device 10. Since the temperature of the gas discharged from the incinerator may vary over a wide range, the temperature of the gas entering the gas cleaning device 10 may be excessive even after passing through the quenching zone. Further cooling is provided by the scrubbing liquid introduced into the cleaning device through conduit 26 thus reducing the moisture content of the gases thereby reducing the fog forming possibility. Consequently, if the temperature of the gas is excessive, the controller 30 will actuate the valve 28 to increase the supply of liquid into the cleaning device 10. The controller 30 will actuate the valve to an open, or flow increasing position until the temperature in the discharge duct 14 decreases to the desired or acceptable level. The controller 30 will conversely actuate the valve 28 in a flow decreasing direction as the temperature of the gas in duct 14 decreases.

During the entire process, the controller 24 monitors the velocity of the gas in the delivery duct 12 as it is discharged from the incinerator. When the output from the incinerator decreases, the controller 24 senses the decrease in static pressure at the tap 25 and causes the motor 22 to actuate the damper 20 in a direction to increase the flow through the recirculating duct 18 to the delivery duct to make up for the decrease in velocity. Consequently, the recirculated gas is again directed through the cleaning device so that additional impurities are removed from the recycled gas. The amount of cleaned gas forced through the recirculating duct 18 varies inversely with the velocity of the gas discharged into the delivery duct 12 from the incinerator.

A system according to the invention thus provides improved cleaning efficiency by recirculating a portion of the gas through the cleaning device when the volume of the gas produced decreases so that additional contaminates are removed from the recirculated gas. Furthermore, the amount of moisture in the cleaned gas can be regulated automatically to control plum and to prevent overload and overheating of the blower. Additionally, the system permits efficient and economic control of the scrubbing liquid as well as efficient and economic employment of the blower and other equipment used in the system.

' While a specific form of the invention has been illustrated and described, it should be understood that the invention is not limited to the exact construction shown but that various alterations and modifications in the construction or arrangement of parts will be possible without departing from the scope and the spirit of the invention.

We claim:

l. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; and flow control means interconnected with said delivery duct, discharge duct and recirculating duct for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct.

2 2. A system as claimed in claim 1 wherein said flow control means includes a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct.

3. A system as claimed in claim 2 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct;

4. A system as claimed in claim 3 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases.

5. A system as claimed in claim 4 further includinga conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in a condition external to said cleaning device.

6. A system as claimed in claim 5 wherein said regulating means comprises a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct.

7. A system as claimed in claim 1 further including a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in a condition external to said cleaning device.

8. A system as claimed in claim 7 wherein said regulating means comprises a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct.

9. A system as claimed in claim 8 wherein said regulating means includes a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases.

10. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; flow control means for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in' said delivery duct, said flow control means including a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct, a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct, and a pressure responsive control for said motor connected with said delivery'duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases; a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in a condition external to said cleaning device, said regulating means comprising a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct and including a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to actuate said valve to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases.

1 1. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed'through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for'causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blowerand its other end connected with said delivery duct; flow control means for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct; a conduitconnected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in a condition external to said cleaning device, said regulating means comprising a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct; said regulating means further including a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases; and said flow control means including a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the i zi fi fifin a l irir fd h aaim 11 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct.

13. A system as claimed in claim 12 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said, damper in said opposite direction when the velocity of gas in said delivery duct increases.

14. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed therethrough, said gas cleaning device having an inlet for receiving gas and anoutlet for discharging gas; a conduit for supplying liquid to said gascleaning device for contact with gas in said cleaning device; and temperature responsive flow regulating means controlling said conduit operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature of gas discharged from said outlet, said flow regulating means comprising a modulating valve operable to vary the flow into said cleaning device in accordance with variations in the temperature of gas discharged through said outlet; a delivery duct connected with said inlet; a discharge duct connected with said outlet; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; and flow control means interconnected with said delivery duct, discharge duct, and recirculating duct for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct.

15. A system as. claimed in claim 14 wherein said flow control means includes a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct.

16. A system as claimed in claim 15 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in'response to increases in the velocity of gas in said delivery duct.

17. A system as claimed in claim 16 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases.

i i i 

1. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; and flow control means interconnected with said delivery duct, discharge duct and recirculating duct for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct.
 2. A system as claimed in claim 1 wherein said flow control means includes a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct.
 3. A system as claimed in claim 2 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct.
 4. A system as claimed in claim 3 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases.
 5. A system as claimed in claim 4 further including a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accoRdance with variations in a condition external to said cleaning device.
 6. A system as claimed in claim 5 wherein said regulating means comprises a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct.
 7. A system as claimed in claim 1 further including a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in a condition external to said cleaning device.
 8. A system as claimed in claim 7 wherein said regulating means comprises a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct.
 9. A system as claimed in claim 8 wherein said regulating means includes a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases.
 10. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; flow control means for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct, said flow control means including a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct, a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct, and a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases; a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; and regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in a condition external to said cleaning device, said regulating means comprising a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct and including a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to actuate said valve to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases.
 11. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed through said cleaning device; a delivery duct connected with said cleaning device for conducting gas to said cleaning device; a discharge duct connected with said cleaning device for conducting gas from said cleaning device; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; flow control means for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct; a conduit connected with said cleaning device for conducting liquid into said cleaning device for contact with gas therein; regulating means controlling said conduit and operable to vary the flow of liquid into said cleaning device in accordance with variations in a condition external to said cleaning device, said regulating means comprising a modulating valve operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature in said discharge duct; said regulating means further including a temperature controller for said valve connected with said discharge duct and operable to actuate said valve to increase the supply of liquid to said cleaning device when the temperature of gas in said discharge duct rises and to decrease the supply of liquid to said cleaning device when the temperature of gas in said discharge duct decreases; and said flow control means including a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct.
 12. A system as claimed in claim 11 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct.
 13. A system as claimed in claim 12 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases.
 14. A system for cleaning gas discharged from an incinerator or the like comprising: a gas cleaning device operable to remove impurities from gas fed therethrough, said gas cleaning device having an inlet for receiving gas and an outlet for discharging gas; a conduit for supplying liquid to said gas cleaning device for contact with gas in said cleaning device; and temperature responsive flow regulating means controlling said conduit operable to vary the flow of liquid into said cleaning device in accordance with variations in the temperature of gas discharged from said outlet, said flow regulating means comprising a modulating valve operable to vary the flow into said cleaning device in accordance with variations in the temperature of gas discharged through said outlet; a delivery duct connected with said inlet; a discharge duct connected with said outlet; a blower for causing gas to move through said discharge duct; a recirculating duct having one end connected with said discharge duct on the downstream side of said blower and its other end connected with said delivery duct; and flow control means interconnected with said delivery duct, discharge duct, and recirculating duct for proportioning flow between said recirculating duct and said discharge duct in accordance with variations in the velocity of gas in said delivery duct.
 15. A sYstem as claimed in claim 14 wherein said flow control means includes a damper movable in one direction to increase the flow through said recirculating duct and decrease the flow through said discharge duct, and movable in the opposite direction to decrease the flow through said recirculating duct and increase the flow through said discharge duct.
 16. A system as claimed in claim 15 wherein said flow control means includes a motor connected with said damper and operable to move said damper in said one direction in response to decreases in the velocity of gas in said delivery duct, and operable to move said damper in said opposite direction in response to increases in the velocity of gas in said delivery duct.
 17. A system as claimed in claim 16 wherein said flow control means includes a pressure responsive control for said motor connected with said delivery duct and operable to actuate said motor to move said damper in said one direction when the velocity of gas in said delivery duct decreases and to actuate said motor to move said damper in said opposite direction when the velocity of gas in said delivery duct increases. 